It is generally known that in the case of a deformation element made of ferromagnetic materials, for instance, of a Ni—Fe alloy, the magnetoelastic effect is able to be evaluated to the effect that the permeability of the ferromagnetic material is a function of the effective mechanical stress on the deformation element. Thus, if tensile or pressure forces act as deformation forces, the magnetic properties of the stressed material changes.
Furthermore, it is customary, for measuring hydraulic pressures in motor vehicles, for instance, in hydraulic brake systems, that as accurate as possible a recording of the pressure is able to be carried out in a signal form that makes possible further processing in an electronic control unit, for instance, in antilock systems.
A pressure sensor is described in European Patent No. EP 0615118 which makes use of a magnetostriction effect of an amorphous magnetic alloy, made up, for instance, of the substances Fe—Si—B—Cr, a deformation element being present, here too, to which pressure is applied. Furthermore, a reference chamber is present which does not experience any pressure application, and two detection coils are present for measuring the permeability change of the magnetostrictive or the magnetoelastic layer, as well as a magnetic feedback path. The actual sensor element itself is made up only of passive components, in this context, and is connected by cable to an evaluation circuit and a drive circuit.
Such a system is also described in Japanese Patent No. JP-2005241567, in which a magnetostrictive element is located in a liquid or gaseous medium that has a pressure applied to it, and in which the permeability change is measured from the outside, through the wall of the pressure element, using a coil.
Moreover, a system is described in PCT International Patent Publication No. WO 2006/117293 in which there is also a pressure element present, the pressure element either being made of a magnetoelastic material itself, being furnished with a magnetoelastic layer, or a magnetoelastic deformation element is situated in the pressure element. The detection of the permeability change takes place, here too, using coils or, alternatively, using Hall sensors in combination with a field-generating element.
It is known in addition, from the last-named related art, that coils and flux guidances may be pushed over the deformation element, whereby a two-part or multiple-part capability of the sensor system is achieved. Consequently, it is known from this, as seen for itself, that a separation of a hydraulic unit and an electronic unit is possible, without having to reserve a plug/contact system for the electrical contacting.
This known form of the separation of the components hydraulic unit and electronic unit has a disadvantage, however, inasmuch as the coil and the flux guidance have to be plugged onto the deformation element, and consequently, during the transition of the flux guidance to the deformation element (magnetic circuit), a gap is created. A change in the gap length during operation, that often cannot be prevented, has the effect of a relatively drastic change in the effective permeability, that may enter into the measuring signal.